The bright long-lived Type II SN 2021irp powered by aspherical circumstellar material interaction (II): Estimating the CSM mass and geometry with polarimetry and light curve modeling

TL;DR

SN 2021irp is interpreted as a canonical Type II SN undergoing strong interaction with a massive, aspherical circumstellar disk. Imaging- and spectro-polarimetry reveal a high intrinsic continuum polarisation around $\sim0.8\%$, indicating an aspherical photosphere generated by disk-like CSM interaction; modelling ties this to a disk with half-opening angle $\theta_{0}\sim30$–$50^{\circ}$ and a mass-loss rate $\dot{M}_{\rm disk}\sim0.035$–$0.1\,M_{\odot}$ yr$^{-1}$, corresponding to a total CSM mass $M_{\rm CSM}\gtrsim2\,M_{\odot}$. The bolometric light curve, treated with a disk-CSM interaction model, supports a normal red supergiant SN ejecta ($M_{\rm ej}\approx10\,M_{\odot}$, $E_{\rm ej}\approx10^{51}$ erg) interacting with a several-solar-mass CSM disk, implying that the extreme mass ejection occurred within decades of explosion, likely due to binary interaction. The results favor a torus-like photosphere and reprocessed radiation from the interaction region, accounting for the broad Balmer lines seen at late times and the lack of strong narrow lines. Overall, the work proposes a cohesive picture for Type II SNe interacting with massive disk CSM and highlights polarimetry as a key diagnostic of aspherical CSM geometry and progenitor evolution in such systems.

Abstract

There is evidence for interaction between supernova (SN) ejecta and massive circumstellar material (CSM) in various types of SNe. The mass-ejection mechanisms that produce massive CSM are unclear, and studying interacting SNe and their CSM can shed light on these mechanisms and the final stages of stellar evolution. We aim to study the properties of the CSM in the bright, long-lived, hydrogen-rich (Type II) SN 2021irp, which is interacting with a massive aspherical CSM. We present imaging- and spectro-polarimetric observations of SN 2021irp. By modelling its polarisation and bolometric light curve, we derive the mass and distribution of the CSM. SN 2021irp shows a high intrinsic polarisation of $\sim0.8$%. This high continuum polarisation suggests an aspherical photosphere created by an aspherical CSM interaction. Based on the bolometric light curve evolution and the high polarization, SN 2021irp can be explained as a typical Type II SN interacting with a CSM disk with a corresponding mass-loss rate and half-opening angle of $\sim0.035$ - $0.1$ M$_{\odot}$ yr$^{-1}$ and $\sim30$ - $50^{\circ}$, respectively. The total CSM mass derived is $\gtrsim 2$ M$_{\odot}$. We suggest that this CSM disk was created by some process related to binary interaction, and that SN 2021irp is the end product of a typical massive star (i.e. with Zero-Age-Main-Sequence mass of $\sim 8-18$ M$_{\odot}$) that has a separation and/or mass ratio with its companion star that led to an extreme mass ejection within decades of explosion. Based on the observational properties of SN 2021irp and similar SNe, we propose a general picture for the spectroscopic properties of Type II SNe interacting with a massive disk CSM.

Figures (8)

• Figure 1: Polarisation spectra of SN 2021irp before and after the ISP subtraction. (a) Total polarisation $P$, Stokes parameters $Q$ and $U$, polarisation angle $\theta$, and signal-to-noise ratio (SNR) before the ISP subtraction at a phase of 203.0 d (black lines). The data are binned to 50 Å per point. The grey lines in the background of each plot are the unbinned total-flux spectra at the same epoch. The ISP is described by $P_{\rm{ISP}}=0.32$ %, $\theta_{\rm{ISP}} = 8.8$° (red lines). The blue hatching shows the adopted wavelength range for the ISP-dominated components. (b) Same as (a), but after the ISP subtraction. The brown and blue lines show the transmission curves of the R band and FILT_815_13. The red hatching shows the adopted wavelength range for the estimate of the continuum polarisation.
• Figure 2: Time evolution of the continuum polarisation in SN 2021irp, compared to those of Type IIn SNe in Bilinski2024. Here, the peak date for SN 2021irp is set to the date of the maximum magnitude in the ATLAS-o-band light curve as 59331.3 (MJD), although the actual peak was not caught by the observations.
• Figure 3: Results for the light curve fitting. (a): Residual between the observed pseudo-bolometric light curve and the best-fit model for each combination of the mass-loss rate ($\dot{M}_{\rm{disk}}$) and the half-opening angle ($\theta_{0}$). (b): Energy conversion efficiency ($\epsilon$) for each best-fit model.
• Figure 4: Time evolution of the luminosity for several well-fitting models. The pseudo-bolometric luminosity estimated with the optical photometry is shown with black open circles, while the total luminosity from the optical and infrared photometry is with red open circles. Model parameters are listed in the figure. We excluded the late-phase data ($t\gtrsim 200$ d; gray hatching) from the light curve fitting.
• Figure 5: Time evolution of the shock shell mass (top left), shock radius (top right), shock velocity (bottom left) and optical depth of the shock shell (bottom right) for several well-fitting models. The adopted model parameters are the same in Fig. \ref{['fig:LC_fitting_bol']}. We excluded the late-phase data ($t\gtrsim 200$ d; grey hatching) from the light curve fitting. The black open circles in the top right panel are the blackbody radii of the optical component from the two blackbody fitting in Paper I.